More haste means you're spending your mana faster on more shadow word pains, and a lot more casts of Mind Flay.

Haste isn't really a mana positive stat. Mastery on the other hand...

Haste is definitely mana-positive. It causes you to have a relatively smaller number of DoT casts in your rotation, as their length does not decrease (at least, in the broad sense, ignoring local irregularities.) Mind Flay spends mana much more slowly than DoTs do.

Haste is definitely mana-positive. It causes you to have a relatively smaller number of DoT casts in your rotation, as their length does not decrease (at least, in the broad sense, ignoring local irregularities.) Mind Flay spends mana much more slowly than DoTs do.

The length of DoTs does decrease between breakpoints, and then snaps back to full length when you hit the point where another tick is added.